Notice that R7 establishes full adjacency only with the Designated
Router (DR) and the Backup Designated Router (BDR). All other routers have a
two-way adjacency established. This is normal behavior for OSPF.

Whenever a router sees itself in a neighbor hello packet, it confirms
bidirectional communication and transitions the neighbor state to two-way. At
this point, the routers perform DR and BDR election. Once DR and BDR are
elected, a router attempts to form a full adjacency with a neighbor if one of
the two routers is the DR or BDR. OSPF routers become fully adjacent with
routers with which they have successfully completed the database
synchronization process. This is the process by which OSPF routers exchange
link-state information to populate their databases with the same information.
Again, this database synchronization process is only executed between two
routers if one of the two routers is the DR or BDR.

OSPF was designed keeping in focus the requirements of large networks.
If all routers formed adjacencies with every other attached router, a large
number of link-state advertisements (LSAs) would be sent over the network. If
n is the number of routers attached to a broadcast
network, there would be n * (n-1) / 2
neighbor pairs. If every pair of neighbors tries to synchronize databases, the
amount of LSAs is enormous. In that scenario, a router floods an LSA to all its
adjacent neighbors, which in turn floods them to all its adjacent neighbors,
and so on. As you can see in this neighbor diagram, if each router has to
synchronize databases with each of its neighbors, each router needs to
establish four adjacencies:

OSPF avoids synchronizing between every pair of routers in the network
by using a DR and BDR. In this way, adjacencies are formed only to the DR and
BDR, and the number of LSAs sent over the network is reduced. Now, only the DR
and BDR have four adjacencies, and all the other routers have two. For this
reason, the routers at the hub of the point-to-multipoint network over
nonbroadcast multiaccess (NBMA) media should be configured as DR/BDR. Refer to
the document Problems with Running OSPF
in NBMA Mode over Frame Relay for more
information.

Sometimes it is desirable for a router to be configured so that it is
not eligible to become the DR or BDR. You can do this by setting the OSPF
priority to zero with the ip ospf priority
priority# interface subcommand. If two
OSPF neighbors both have their OSPF interface priority set to zero, they
establish two-way adjacency instead of full adjacency.

The topology below provides an example. There are three routers
connected via Frame Relay. The Frame Relay interfaces are defined as broadcast,
but only the router with a connection back to the main network is eligible to
be the DR. The other two routers have their interface priorities set to zero,
so they are not eligible to become the DR or BDR. Although they do become
neighbors, they only reach two-way state.